Cathode ray tube apparatus



Nov. 4, 1941.

CATHODE Filed Dec. 50, 1958 W. J. POCH RAY TUBE'APPARATUS 2 Sheets-Sheet 2 1 G d c/Mm1crEH/.swc of' 'rz/BL' 16 'ya :A E I Gir/D vaL'ms' l *TB/ms I ,zcx/.s 'l

:inventor I v Waldemar J Poeh zu;v y I Gttorneg Patented Nov. 4, 1941 CATHODE RAY TUBE APPARATUS A Waldemar J. Poeh, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1938, Serial No. 248,570

(Cl. Z50-27) 4 Claims.

My invention relates to cathode ray tube apparatus and particularly to protective and/ or blankingcircuitsfor cathode ray -tubes in a television .l system.

The invention will be described with respect to va rtelevision transmitter of the type which utilizes a cathode ray pickup vtube or iconoscope. In transmitters of this type, it is desirable to block the cathode ray during the horizontal re- -turn line period and during the vertical return period while the iconoscope mosaic screen is being scanned. Itis also desirable to provide protective means for the purpose of avoiding damage to the mosaic screen in the event of failure of the cathode ray defiecting means. i

` It is accordingly an object Vof my invention to provide an improvedl circuit for blanking or blocking a cathode ray duringl the return period following its useful deiiecting period.

It is a further object of my invention to provide an improved protective circuit for a cathode ray tube. l f

`Itis a further object of my invention tol provide an improved 'circuitwhich functions both as a cathode ray tube protective means and as a returnlne blanking means.

The invention will be better understood vfrom the following description vtaken in connectionl with the accompanying drawings iny which- Figure lis a Vcircuit diagram of a'preferred embodiment of my invention, c

' Figures 2, 3 and 1 are curves which are referred to in explaining the invention, andk Figure 5 is a diagram showing'the action ofV one'of the amplifier tubes inFlgure `1.

' Referring to Fig. l, my invention is shown appliedr to a television transmitter comprising a cathode ray pickup tube I of the type known as the iconoscope. The tube V.I comprises a highly evacuated envelope having thereinvan indirectly heated cathode 2, aA control electrode 3,[a first anodel, a second anode 6 and a mosaic screen 1 upon which there is projected the picture to be transmitted. The screen 'I in' its most common form is made up of 'a large number of minute photoelectric or capacity elements which are insulatedfrom each other and from a metallic signal-plate 8. c

Thefcathode ray is caused to scan the screen 1 by'rneans of deflecting ycoils I I and I2 which deflect the ray horizontally and vertically, respectively. Saw-tooth vdeflection is employed, sawtooth current, which recurs ata comparatively high frequency, being forced through the coils I I by means of a horizontal deflecting circuit I3, and

saw-tooth current, which recurs at a comparatively low frequency, being forced through the coils I2 by means of a vertical delecting circuit 14.

As indicated at I5 in Fig. 2, the voltage across the horizontal deflecting coils II is substantially a rectangular voltage impulse during the returnv line period. However, the voltage across the vertical deflecting coils I2, while it contains an impulse component, is close enough to a saw-tooth wave so that it may be considered as such for the present purpose. Thus the voltage across coils I2 may be represented as in Fig.3. Whilethe particular wave shape across a specifirl defie'cting coil is not important so far as the invention is concerned, it may be noted that the reason for the difference in the voltage wave shapes across coils II and I2 is'that at the high horizontal` deiiecting frequency the coils II have high inductive reactance as compared with their resistance, whereas at the low vertical deflecting frequency thev coils 'I2 have low inductive reactance as compared with their resistance.

The voltages appearing across deflecting coils II Aand I2 are applied through conductors I6 and I'I to the input circuits of amplifier tubes IB and I9, respectively, which have a common plate circuit connected to the control electrode 3vfor supplying blanking impulses thereto during normal operation and for supplying a high protective bias in the event of failure of either the' horizontal 0r vertical' deflection.

First, considering the impulses taken from the horizontal defiecting coils Il, they are impressed across a voltage divider resistor 2I through a coupling condenser 22. yVoltage impulses from divider 2I are impressed through a grid condenser 23 and a conductor 24 across the grid 26 and the cathode 21 of the tube I8. It will be notedthat the cathode-is held at A.C.l ground potential by a bypass condenser v25.

A grid leak resistor 28 is provided which has suiiiciently `high resistance with respect to the grid condenser 23 to. provide grid leakbias on the tube I8 due to the positive voltage impulses from the deflecting coils II driving the grid 26 positive periodically.

The plate 29 of the tube I 8 is supplied with an operating voltage through a plate resistor 30, the voltage being taken from a potentiometer resistor 3l connected across a portion of the voltage divider 32 of the power supply unit. Potentiometer 3I is provided with bypass condensers 33 and 34.

The voltage drops across plate resistor 30 and across the right-hand portion of potentiometer 3| are applied through conductors 36 and 31 to the control electrode 3 of the cathode ray tube as a biasing potential.

Thus it will be seen that each voltage impulse applied to the grid of tube I8 causes a negative impulse to be applied to the control electrode 3, this negative impulse being of sufficient amplitude to block the cathode ray during the return line period. It will also be seen that in the event of failure of the horizontal deection the negative bias on the grid I8 is reduced (becomes zero in the circuit shown) whereby the plate current through resistor 3D increases to apply a high negative bias to control electrode 3, this high bias being great enough either t'o block the cathode ray or to so reduce its intensity that it will not injure the mosaic screen 1 whenit is stationary thereon or when it is being deflected in one direction only.

It will be understood that the above-mentioned grid leak biasing is obtained due to grid current charging condenser 23 each time the grid 26 is driven positive by a voltage impulse. At the end of each charge, the condenser 23 partially discharges through the grid leak resistor 28, the timeconstant being such that this discharge between successive incoming impulses is small whereby the biasing voltage is substantially a constant value.

Figure 5 illustrates the operation of the ampliiier tube I8. The voltage impulses, having the A.C. `axis indicated, are'applied to the tube with sufficiently low amplitude to cause the greater part of. each impulse to appear in the plate circuit. It will be noted that peaks of impulses I5 drive the grid positive to provide the bias indicated on the drawings. The impulses appearing in the plate circuit of tube I8 vpreferably 'have several times the amplitude necessary for driving the cathode ray tube I beyond cut-off. Thus the impulses I5 are effective for blanking the cathode ray for a period determined by the width of the impulses at a point a considerable distance down from the top of each impulse as indicated by the dotted lines in Fig. 2.

In its preferred form, the invention is applied to the vertical deflecting 'circuit as well as to the horizontal deflecting circuit. As shown in the drawings, the voltage from across deflecting coils I2 is applied through the conductor I1 across what may be referred to as a diierentiating circuitthis circuit comprising a condenserM anda resistor 42 in series. By choosing proper values, the voltage appearing across the resistor42 may be given substantially an .impulse wave form as shown at 40 in Fig. 4. This voltage is impressed across the grid 43 and cathode /44 of the tube I9 through a grid condenser 46 and through a connection which may be traced through ground, the bypass condenser 25 and a conductor 41 to the cathode 44.

In order to provide grid leak ybiasing for the tube I9, a grid leak resistor 48 is given high enough resistance to -prevent the charge on the condenser 43 from leaking 01T too fast between successive voltage impulses. This action is the same asdescribed in vconnection with the biasing of the tube I8-,

If desired, a series of resistor i9 and a shunt condenser 5I may be provided in order to reduce the possibility .of horizontal cross talk from affecting the operation of the tube I 9. The resistor 49 also helps Vto fla-tten off the peaks of the impulses shown in Fig. 4 to make them more suitable for blanking.

The plate 52 of the tube I9 is supplied with an operating voltage through the plate resistor 30 whereby the plate current of this tube as Well as that of tube I8 ows therethrough. Thus, a decrease in bias on the grid of either tube I8 or I9 Will increase the negative bias on the control electrode 3.

The wave-shaping elements 4I and 42 are given the proper values to put a vertical blanking im- 'pulse on the cathode ray tube of the proper width or duration to block the cathode ray during the vertical return line period. If the deflecting circuit design is such that the voltage across coils I2 is substantially an impulse, the differentiating circuit 4 I-42 is unnecessary.

In the example illustrated, the vertical blanking impulses 4I! are of suflicient amplitude to drive the cathode ray tube I beyond cut-off a substantial amount, whereby the effective blanking width of these impulses is that indicated by the dotted lines in Fig. 4.

It will be apparent that in the circuit illustrated the adjustment is such that, when the cathode ray is being deected in both directions, a normal small operating bias is applied to the control electrode 3, the current through resistor 3B being small at this time. In the event of failure of either defiecting circuit, the bias on an amplifier tube is removed and the current flow through the plate resistor 36 increases suiciently to block the cathode ray. Also, during normal operation, the impulses shown in Figs. 2 and 4 are applied to the control electrode 3 with suflicient amplitude to block the cathode ray during the return line periods.`

On the drawings, the values of certain resistors and condensers have been `indicated in ohms, megohms and microfarads, merely by way of example.

From the foregoing, it will be seen that I have provided an improved circuit for protecting a cathode ray tube as well as a simple circuit which functions both for blanking the cathode ray and for giving the desired protection.

I claim as my invention:

l. The combination of cathode ray producing means having a control electrode for controlling the intensity of said ray, a deflecting device for deflecting said ray, deflecting circuit means for applying a periodic deiiecting voltage across said deflecting device for deflecting said ray, and means controlled by said periodic voltage both for blocking said cathode ray during the return line period and for vapplying a substantially steady negative biasing voltage to said control electrode of suflicient amplitude to substantially reduce the intensity of said ray in response to failure of said deiiecting circuit means.

2. The combination of cathode ray producing means having a control electrode for controlling the lintensity vof said ray, a deflecting device for deiiecting said ray, a delecting circuit for applying a periodic deflecting voltage across said deflecting device for deflecting said ray, an amplifier tube having a cathode, a control grid and a plate, means for applying said periodic voltage across said cathode and control grid through a grid -condenser with said voltage being of positive polarity at the control grid, a grid leak resistor connected between said cathode and said control grid, said grid condenser and said grid leak resistor having such relative values a substantial steady biasing voltage is applied to said control grid due to a'grld leak biasing action, means for applying an operating voltage to said plate through a plate resistor, and means 'for connecting the control electrode and cathode of said cathode ray tube across said plate resistor through direct current connections whereby blanking impulses are applied to said control electrode through said tube and whereby an increasednegative voltage is applied to said control electrode to reduce theA intensity of the cathode ray in response to failure of said deecting circuit.

3. Cathode ray tube apparatus comprising a cathode ray tube including means for producing a cathode ray and having a control electrode for controlling the intensity of said ray, horizontal and vertical deflecting devices for deflecting said ray, horizontal and vertical deflecting circuits for applying periodic deiiecting voltages across said horizontal and vertical deecting devices, respectively, for deflecting said ray to scan a rectangular pattern, a pair of amplifier tubes each having a cathode, a control grid 'and a plate, means for applying one ofsaid periodic voltages across the cathode and control grid ofl one of said tubes through a grid condenser with said voltage being of positive polarity at the f control grid, means for applying the other of tube through a common plate resistor, and means for connecting the control electrode and cathode of said cathode ray tube across said plate resistor through direct current connections whereby blanking impulses are applied to said control electrode through said tube and whereby an increased negative voltage is appliedl to said control electrode to reduce the intensity of the cathode ray in response to failure of either of said deflecting circuits.

4. The combination of cathode ray producing means having a control electrode forcontrolling the intensity of said ray, a deecting device for deflecting said ray, deflecting circuit means for applying a periodic deflecting voltage across said deflecting device whereby there is produced a deecting field having a useful deflecting portion and a return line portion, an amplier tube having a control grid and having an input circuit which includes said grid, said amplifiertube having a plate circuit which includes a plate resistor, means for obtaining blanking pulses fromV said defiecting circuit and for applying them to said input circuit, each of said blanking pulses having a duration approximately equal to the duration of said return line portion and having suflicient amplitude to pass through vsaid amplifier tube, means for generating a negative biasing voltage for said grid in response to the application of said blanking pulses to said input circuit, and means for connecting the control electrode and the cathode of said cathode lray tube across said plate resistor through direct current connections whereby blanking impulses are applied to said control electrode through said tube and whereby an increased negative voltage is applied to said control electrode to reduce the intensity of the cathode ray in response to failure of said deflecting circuit. WALDEMAR J. POCIjI. 

